It is well known that fuel impurities can adversely affect the performance of jet engines. The mechanism of the formation of deposits from thermally stressed jet fuels is a complex process involving several consecutive reactions steps. In the most general case, deposits form as the result of a fuel oxidation, of which the first step in the mechanism is the formation of peroxides. The thermal decomposition of peroxides and the ensuing chain propagation reactions lead to the formation of several oxygenated compounds and free radical species. The free radicals react with dissolved oxygen in the fuel to form relatively stable alkylperoxyl radicals; because these less reactive radicals tend to build up in the fuel, they have a greater probability of recombining with each other to form species that have slightly more than twice the molecular weight of the average fuel molecule. These higher molecular weight species, commonly known as gums, which are relatively insoluble in the fuel, contain high concentrations of oxygen and other heteroatoms such as sulfur and nitrogen that may be present in the fuel.
It is generally believed that the insolubles/gums are the precursors to deposit formation. Since deposit formation depends on parameters other than chemistry such as the flow conditions, mass transport and surface activity, thermal stability of a fuel seems to be best indicated by the fuel's tendency to form gums. A temperature dependent global rate constant for the formation of gums could then become a defined fuel property for thermal stability and could be used for predicting the potential deposit formation in aircraft fuel systems at specified temperature and flow conditions.
To determine the global rate constant for the formation of insolubles in fuels, the rate of gum formation needs to be measured under controlled conditions, specifically, constant temperature. However, the actual measurement of the rate of formation of gums hinges upon the current capability to measure gum concentration in jet fuels. Currently, the ASTM D381 method using the stream jet evaporation technique is the only established method of measuring the concentration of gums in jet fuels. This analysis method involves a large sample size, lengthy analysis, and yields poor accuracy. Furthermore, its intrusiveness tend to discourage the use of the D381 method in an examination of the kinetics of gum formation in jet fuels.
The deposits that form in aircraft fuel systems include soft gums, strongly adhering lacquers, and varnishes. Sometimes there are very hard, brittle substances resembling coke. When deposits are formed by autooxidation of jet fuel, they have oxygen concentrations much greater than deposits from the thermally unstressed fuel, and their hydrogen-to-carbon ratio is lower than that of the original fuel. If the unstressed fuel contains hetroatoms such as sulfur and nitrogen, these relatively polar species are concentrated in the deposit.
There is very little quantitative data on the composition of fuel-system deposits formed from jet fuels. Furthermore, there is little data available on the compositions of gums formed by the autooxidation of fuels. The data on gums are important because they are precursors of deposit formation and thus provide an indication of deposit composition. Data pertaining to the compositions of gums formed by the autooxidation of heating oil and gasoline show that the polar components, including the hetroatoms oxygen, sulfur, and nitrogen, are highly concentrated in the gum, although they are usually present in only minute amounts (&lt;&lt;1%) in the original fuel.
In view of the difficulties in measuring gums in jet fuels, spectroscopic methods of analysis have been explored. In the examination of fuels that contain gums, it has been found that the gums seem to always cause the fuel to have a slight brandy color that is characteristic of light absorption in the blue region of the visible spectrum. Other similar observations have been reported, such as the work by Bhan et al. reported in "Color Change/Sediment Formation in Marine Diesel Fuels, Task II," NIPER-B06710-2 (1986), which suggested a correlation between color change and sediment formation in marine diesel fuels.
The fact that pristine hydrocarbon fuels do not absorb light in the visible region of the spectrum, but gums have a finite absorption there, suggests that a spectroscopic method of measuring gums is possible. The first method that comes to mind is simply light absorption in the blue region (ca. 450 nm) of the spectrum as mentioned above. Although absorption would appear to work in principle, it does not appear to be strong enough to indicate that a highly sensitive method of measuring gums could be developed. A more sensitive method requires a laser diagnostic technique, such as that discussed hereinbelow.